In some cases, an engine of a motorcycle or the like may be provided with a secondary air introduction device configured to supply air into an exhaust port by using a negative pressure in the exhaust port. According to the secondary air introduction device, it is possible to combust an unburnt gas in exhaust air to be discharged from a combustion chamber by the air introduced into the exhaust port. Thereby, it is possible to increase a temperature of the exhaust air upon start of the engine and the like, thereby activating catalyst in early stage.
The secondary air introduction device is configured by forming a passage for introducing the air from an air cleaner of an air intake system into the exhaust port, forming a valve chamber in the middle of the passage and providing a reed valve in the valve chamber, for example. In the secondary air introduction device, when a negative pressure is generated in the exhaust port by pulsation of the exhaust air, the reed valve is opened by the negative pressure, so that the air is sucked from the air cleaner into the exhaust port (refer to Patent Document 1).
Patent Document 1: Japanese Patent No. 3,330,019B
In the secondary air supply device, in order to open the reed valve by the negative pressure in the exhaust port and to thereby take the air into the exhaust port, the valve chamber and the exhaust port are configured to communicate with each other by a passage formed in the cylinder head and the like, for example. For this reason, for example, immediately after the exhaust valve is opened and the exhaust air is thus discharged from the combustion chamber, a part of the exhaust air may flow back through the passage between the valve chamber and the exhaust port and directly collide with a valve body of the reed valve provided in the valve chamber.
When the high-pressure exhaust air having flowed back directly collides with the valve body of the reed valve, the valve body is vibrated. By the vibration, the deterioration of the valve body may be accelerated or the valve body may be damaged. Also, the high-temperature exhaust air having flowed back may be introduced into the air cleaner through a gap between the valve body and a valve seat, which is formed due to the vibration of the valve body. In this case, a seal member formed at a part, on which the valve body is seated, of the valve seat of the reed valve may be deteriorated or eroded in early stage by heat of the exhaust air.
Also, the reed valve has a stopper configured to restrain an amount of movement of the valve body in a direction of separating from the valve seat. Depending on an angle at which the passage configured to communicate the valve chamber and the exhaust port each other is connected in the valve chamber, a mounting position of the reed valve in the valve chamber, and the like, the exhaust air having flowed back may directly enter between the valve body of the reed valve and the stopper and flow at high speed between the valve body and the stopper, so that the valve body may be floated from the valve seat by a stream of the exhaust air. For this reason, the high-temperature exhaust air may flow back toward the air cleaner beyond the reed valve, thereby deteriorating or damaging a piping and the like provided on a path between the air cleaner and the valve chamber, for example.
Patent Document 1 discloses a configuration where a heat shield plate is provided in a region of the exhaust port-side in the valve chamber having the reed valve provided therein. The heat shield plate is formed of alumina, and the heat shield plate is provided with a plurality of vent holes. The exhaust air having flowed back passes through the heat shield plate and reaches the reed valve. Patent Document 1 discloses that when the exhaust air passes through the heat shield plate, the heat of the exhaust air is dissipated by the heat shield plate and the temperature of the exhaust air is thus lowered.
However, as shown in FIG. 6 of Patent Document 1, a substrate of the heat shield plate is formed with the plurality of vent holes over the entire region. For this reason, the exhaust air having flowed back may pass through the vent holes and directly collide with the valve body of the reed valve, thereby vibrating the valve body. Alternatively, the exhaust air having flowed back may pass through the vent holes and directly enter between the valve body and the stopper of the reed valve, thereby floating the valve body.